This invention relates generally to apparatus for injecting fuel into internal combustion engines, and more particularly to an improvement in fuel supply on engine start with an electronic fuel injection apparatus.
Internal combustion engines mounted on motor vehicles or the like are widely controlled electronically in recent days so that the quantity of fuel to be supplied to an engine is computed by a microcomputer in accordance with operating conditions of the engine. The so called electronic fuel injection (EFI) control unit which controls the opening duration of fuel injection valve(s) is becoming popular. In such an electronic fuel injection control apparatus, the operation of a microcomputer used for comuting the fuel injecting duration has to be normal, but the microcomputer is apt to suffer from undesirable influence caused from voltage fluctuation of the power source.
Especially when an engine starter is operated, the voltage of the power source, i.e. a battery mounted on a motor vehicle, drops to a considerable extent since a large current flows into the starter motor. Therefore, in the case that the battery is deteriorated or in poor condition of low ambient temperature, the voltage of the battery sometimes drops below a value where the operation of the microcomputer cannot be ensured on engine start.
To ensure accurate operation of the microcomputer on engine start irrespective of the dropping of the battery voltage, various measures have hitherto been devised. According to one conventional electronic fuel injection control apparatus, an additional fuel injection valve, which is called start injector, is provided so that fuel is supplied to the engine even if the battery voltage is low. This start injector is provided to an intake pipe and is arranged to be responsive to a time switch using a bimetallic element so that fuel is supplied to the engine for a given period of time on engine start. According to another conventional electronic fuel injection apparatus disclosed in Japanese Patent Provisional Publication No. 58-217737, a backup memory is used for prestoring fuel injection duration suitable for engine start, and when the battery voltage drops below a given voltage, the fuel injection duration from the backup memory is used in place of the results of operation performed by the microcomputer.
However, these conventional techniques suffer from the following problems:
(A) When the above-mentioned start injector is used for supplying fuel on engine start, a separate electrical system and a fuel supply system are necessary in addition to the normal fuel injection system, and thereby the structure of the entire fuel supply system becomes complex. As a result, the reliability of the entire system is apt to be lowered, while the number of manufacturing processes increases resulting in a cost increase. Furthermore, the amount of fuel to be injected is unequivocally determined by the time switch, and therefore, precise control in accordance with starting condition of the engine, such as engine coolant temperature or the number of times of fuel injections, cannot be performed. This also applies to the other method of fuel injection using the backup memory.
(B) Since the starter motor receives a maximum load when one of the cylinders of the engine is in the last part of its compression stroke, the battery voltage drastically fluctuates in correspondence with load variation. Therefore, the battery voltage may fluctuate between a voltage with which the microcomputer can normally operate and another voltage with which the microcomputer cannot normally operate. As a result, the entire microcomputer is reset to an initial state each time the battery voltage drops below a given voltage, and therefore, the microcomputer always starts operating from its initial state whenever the battery voltage is restored. Accordingly, when the battery voltage drops below the given voltage to reset the microcomputer before or in the middle of necessary computation of fuel amount to be injected, accurate fuel amount required for engine start cannot be obtained.